Synthetic papers may be classified into two different types: one with a fibrous structure like cellulose paper, with synthetic fibers made from polyamides, polyester, or polyolefins replacing the cellulose fibers; and one in which a film is directly extruded from a thermoplastic polymer. These products differ considerably in their structure from cellulose paper and thus present a variety of different problems.
Extruded film is produced either by the cast film process or the blown film process in which the melt is forced through a circular die. A variant of the blown film process is the extrusion of foamed film in which through the addition of blowing agents (usually low boiling hydrocarbons) it is possible to produce foamed film from a variety of polymers. Two methods are used for the biaxial stretching of cast film: the simultaneous stretching process, in which the stretching is carried out in both directions at the same time, and the two-stage process, in which the two stretching operations are carried out consecutively.
Extruded films have a smooth surface which depends more on the state of the chill rolls than on the material itself. There are no cavities with capillary activity such as between the fibers of cellulose paper or synthetic fiber webs. This combination of smooth surface, low absorbing power, and non-polar structure frequently makes it difficult to print on polymer films: drying times are long, and the adhesion of the printing ink is poor.
Lunk and Stange in 1971 in Angewandte Chemie International Edition, volume 10, pages 287-294 stated that one type of surface treatment to improve the printability of extruded films is the application of a pigment coating. In the paper industry, the improvement of the surface of cellulose paper with a pigment coating had been known for some time. Typical binders for these coatings were casein, polyacrylates, or copolymers of butadiene and styrene. China clay, barium sulfate, chalk, or titanium dioxide were used as pigments. In spite of their closed, smooth surface coated papers were sufficiently absorptive for letterpress and offset printing because of their high content of mineral filler. Coated polymer films had similar properties; they could also be used for all printing processes, and the print quality was equal to that of coated papers. However, the demands of the printing industry have increased since 1971 requiring papers with ever higher printability.
GB 2 177 413A discloses a coating composition for a plastics substrate and comprising an aqueous system containing polymeric binder, pigment and antistatic agent characterised in that the binder contains carboxyl groups, that the binder:pigment dry weight ratio is in the range 15:100 to 50:100, preferably 22:100 to 35:100, the antistatic agent comprises a water soluble ionic compound present in an amount corresponding to a dry weight:pigment ratio of from 0.4:100 to 2.5:100 and in that the composition also contains an insolubilizing agent which is capable of reacting with carboxyl groups of the binder on drying the composition to insolubilize (as hereinbefore defined) the binder.
U.S. Pat. No. 5,397,637 discloses a thermoplastic resin film with satisfactory printability having a coated layer comprising a quaternary ammonium salt copolymer comprising (a) a structural unit represented by formula (I):
wherein A represents —O— or —NH—; R1 represents a hydrogen atom or a methyl group; R2 represents an alkylene group having from 2 to 4 carbon atoms or —CH2CH(OH)—CH2—; R3, R4, R5, and R6, which may be the same or different, each represents an alkyl group having from 1 to 3 carbon atoms; R7 represents an alkyl group having from 1 to 10 carbon atoms or an aralkyl group having from 7 to 10 carbon atoms; n represents an integer of from 1 to 3; and X represents a chlorine atom, a bromine atom or an iodine atom, (b) a structural unit represented by formula (II):
wherein R8 represents a hydrogen atom or a methyl group; and R9 represents an alkyl group having from 1 to 22 carbon atoms, an aralkyl group having from 7 to 22 carbon atoms or a cycloalkyl group having from 5 to 22 carbon atoms, and (c) a structural unit derived from a monomer copolymerizable with monomers providing structural units (a) and (b) at a (a):(b):(c) weight ratio of 30 to 70:30 to 70:0 to 40.
JP 11-107194 discloses an offset-printable synthetic paper characterized by providing at least one side of a synthetic substrate with 1-7 g/m2 of a coating layer comprising 20 to 50 wt % of polyvinyl alcohol, 20 to 50 wt % of a polyurethane resin and 20 to 50 wt % of a silica pigment, with it being preferred that the polyurethane resin is a carboxyl-containing crosslinkable polyurethane resins or a polyester-based polyurethane resin and the particle size of the silica pigment is 2-5 μm, and the synthetic paper has 20-50% opacity determined in accordance with the JIS P 8138.
U.S. Pat. No. 6,300,393 discloses a paper coating composition comprising (a) at least one binder containing a plurality of functional groups capable of reacting with carboxylic acid anhydride moieties to form chemical bonds, and (b) at least one insolubilizing agent chosen from the froup consisting of compounds containing a plurality of carboxylic acid anhydride moieties per molecukar entity.
WO 03/033577A discloses a printable, opaque coated sheet, suitable for use in contact and/or association with foodstuffs, the coating comprising by dry weight of the coat: (a) from about 30% to about 70% of a copolymer of an ethylenically unsaturated acid; (b) from about 30% to about 70% of an inert particulate filler; where the coating is substantially free from any reactive binder and/or cross-linking agent. WO 03/033577A further discloses that the ethylenically unsaturated acid is preferably neutralised by ammonia and not metal cations, since this has the advantage that on drying ammonia evaporates and acid groups become less moisture sensitive compared to metal cation stabilised ethylene acrylic acid (EAA) copolymer dispersions.
US 2004/0146699A1 discloses a composite plastics sheet which is printable on at least one surface, and comprises a base layer and optionally a printable layer forming the printable surface, characterized in that the base layer has a density of 0.65 g/mL or less, or, if there is no separate adhesive layer on the surface opposite the printable surface, a density of 0.7 g/mL or less.
US 2006/0257593A1 discloses a printing sheet comprising a substrate and, on at least one side of the substrate, an image receptive coating layer with a cumulative porosity volume of pore widths below 200 nm as measured using nitrogen intrusion methods of more than 0.006 cm3 per gram paper.
EP 1 743 976A1 discloses a coated printing sheet for sheet-fed offset printing with an image receptive coating layer on a paper substrate, characterized in that the image receptive coating layer comprises a top layer and/or at least one second layer below said top layer, said top and/or second layer comprising: a pigment part, wherein this pigment part is composed of 0 to 99 parts in dry weight of a fine particulate carbonate and/or of a fine particulate kaolin 1 to 100 parts in dry weight of a fine particulate silica and a binder part, wherein this binder part is composed of: 5-20 parts in dry weight of binder and less than 4 parts in dry weight of additives.
As the foregoing present certain disadvantages, a need exists for an improved paper for use in printing processes.